Such motors usually have a rotor excited by a permanent magnet, and a stator having a stator winding, to which winding energy is delivered during operation in order to drive the rotor. The rotor can also be externally excited, i.e. by means of a solenoid.
Depending on its design, a motor of this kind can have a suitable number N of phases, e.g. N=1, 3, >3; and it can have a suitable number of strands, e.g. a single-phase motor having one or two strands, a three-phase motor having three or six strands, etc. The motor can have one or more rotor position sensors to sense the rotor position, or it can operate partly or entirely according to the so-called “sensorless” principle.
If a motor of this kind is designed for operation over a wide voltage range, e.g. from 20 to 80 V (also referred to as a “wide voltage range motor”), it is then no longer possible to design the winding so that at the operating rotation speed, the voltage induced by the rotor in the stator winding corresponds approximately to the motor's operating voltage. This conventional design would produces a current limiting effect that is inherent in the motor, but in addition to an approximately constant rotation speed, this also requires an approximately constant operating voltage, which cannot exist with a wide voltage range motor.
In such a motor, the induced voltage is therefore often well below the operating voltage; this is referred to in technical jargon as a “sharp winding,” i.e. such a winding has a small number of windings made of thick wire, and an electronic current limiter must therefore be provided in order to limit the motor current that flows into this “sharp winding.” This applies in particular to startup, when the induced voltage has a value of zero, and a low-impedance winding arrangement can therefore very quickly cause high motor currents if the latter are not limited in an appropriate fashion (i.e., above all, very rapidly). The same applies to a motor of this kind that is running at a high operating voltage and stalls.